The present invention relates to a method of video scene composition from a set of graphic elements and primary video objects.
This invention may be used in any video coding system for improving, for a viewer, the visual reading comfort of graphic elements.
With the emergence of multimedia applications such as interactive television or the Electronic Program Guide (EPG), image quality has become an important aspect. Indeed, having simultaneously a good resolution and a large display format of pictures, videos and graphic elements is now required by end users so that they can communicate and interact with such applications while having a maximum visual comfort. More and more, the content of such multimedia applications is composed of a primary video content into which additional information is inserted. Such additional information may correspond to answers to end user requests or end-user graphic personalization resulting in the mapping of two-dimensional (2D) graphic elements in video frames of said primary video content, such as text or geometric patterns.
U.S. Pat. No. 5,877,771 describes a method and apparatus for 2D texture mapping for providing a richer surface detail in a displayed frame. To this end, multi-resolutional texture data for a destination pixel in the frames of the primary video is super-sampled at horizontal and vertical screen space sampling rates based on the local rate of change in texture. If graphic elements are considered as texture, such a method can be used for the mapping of 2D graphic elements on video frames.
It is an object of the invention to propose an improved and cost-effective method of video scene composition from 2D graphic elements and video objects which allows obtaining a good quality of said 2D graphic elements in the video scene.
Indeed, the prior art method has strong limitations. First, this method does not take into account that the mapping of 2D graphic elements on the primary video frames must be combined with an upscaling of said primary video frames if a larger format is required for the resulting output video frames. Thus, in this context, the quality of 2D graphic elements may be degraded because of aliasing, which results in coarse and jagged symbols not legible to viewers. Secondly, this method remains expensive since it requires a large amount of processing power for the mapping operation.
The method according to the invention provides a solution to the problems posed by the limitations of the prior art method. This method renders it possible to obtain upscaled video frames including 2D graphic elements from primary video objects of smaller format while ensuring that no degradation of said 2D graphic elements will take place. To this end, the method according to the invention is characterized in that it comprises:
a rendering step of said primary video objects for providing rendered video frames in a first format,
an upscaling step of said rendered video frames for providing upscaled video frames in a second format,
a rendering step of graphic elements for providing rendered graphic elements in said second format,
a mapping step of said rendered graphic elements on said upscaled video frames, for composing frames defining the video scene.
This method profits the fact that the output video is composed of two sets of distinct data relating to video objects and 2D graphic elements. Thus, separate and parallel processes are performed on video objects and on 2D graphic elements. The first process consists of composing frames from video object frames, said frames being upscaled to the desired output video format afterwards. The second process consists in directly rendering the 2D graphic elements in said output format, using a drawing algorithm which ensures that no degradation of said 2D graphic elements will take place. In a final step, rendered 2D graphic elements are mapped on the upscaled video frames. This method generates upscaled video frames including 2D graphic elements of good resolution as compared with an upscaling performed on video frames including 2D graphic elements, which leads to upscaled video frames including degraded 2D graphic elements.